<!DOCTYPE html>
<html lang="en">
<head>
	<meta charset="UTF-8">
	<meta name="viewport" content="width=device-width, initial-scale=1.0">
	<title>Document</title>
	<style>
		.outer {
			width: 800px;
			height: 400px;
			padding: 10px;
			background-color: gray;
		}
		.box {
			font-size: 20px;
			padding: 10px;
		}
		.box1 {
			background-color: blue;
		}
		.box2 {
			background-color: red;
			float: left;
		}
		.box3 {
			/* height: 200px; */
			background-color: green;
		}
	</style>
</head>
<body>
	<div class="outer">
		<div class="box box1">盒子1</div>
		<div class="box box2">盒子2</div>
		<div class="box box3">盒子3</div>
	</div>
</body>
</html>